With medical implants which are to be fastened to bone using a bone screw, it is often necessary to prevent backward migration of the screw from its assigned position, so that it cannot pose a danger to adjacent organs. It is not enough, as is known in the prior art, to secure the screws against turning in the direction of unscrewing, because, if their thread turns are engaged loosely in the bone substance, there is a danger of their moving back axially without any appreciable turning. It is therefore known to secure the screws by providing the implant with covering arrangements which, after the screw has been screwed in, are placed over the screw head in order to prohibit its backward movement. Such covering arrangements, however, take up additional space, which can sometimes be undesirable.